It remains a major challenge to directly determine nanoparticle surface structures, because of the lack of analytical tools that are currently available. (Alivisatos, A. P., J. Phys. Chem., 100:13226-13239 (1996)). Early nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) studies of CdSe nanocrystals prepared in coordinating solvents such as tri-n-octylphosphine oxide and tri-n-octylphosphine, suggested these coordinating solvents are datively bound to the surface of a nanoparticle. (Bowen-Katari, J. E. et al., J. Phys. Chem., 98:4109 (1994); Becerra, L. R. et al., J. Chem. Phys., 100:3297-3300 (1994)). However, assigning the broad NMR resonances of surface-bound ligands is complicated by significant concentrations of phosphorus-containing impurities in commercial sources of tri-n-octylphosphine oxide (1), and XPS provides only limited information about the nature of the phosphorus containing molecules in the sample.
More recent reports have shown that the surface ligands of CdSe nanocrystals prepared in technical grade tri-n-octylphosphine oxide, and in the presence of alkylphosphonic acids, include phosphonic and phosphinic acids. (Kopping, J. T.; Patten, T. E. J., Am. Chem. Soc., 130:5689-5698 (2008); Wang, W.; Chem. Mater, 19:2573-2580 (2007)). These studies do not, however, determine if these ligands are bound datively, as neutral, L-type ligands, or by an X-type interaction of an anionic phosphonate/phosphinate moiety with a surface Cd2+ ion. (Puzder, A., et al., Nano Lett., 4:2361-2365 (2004); Manna, L. et al., J. Phys. Chem. B, 109:6183-6192 (2005)). Answering this question would help to clarify why ligand exchange with such particles does not proceed generally as expected based on an L-type ligand model. (Kuno, M. et al., J. Chem. Phys., 106:9869-9882 (1997); Reiss, P. et al., Nano Lett., 2:781-784 (2002); Wang, Y. A. et al., J. Am. Chem. Soc., 124:2293-2298 (2002)).
In general, chemical modification of nanocrystal surfaces using ligand exchange processes is desirable. For example, nanocrystals with modified surfaces can be used in biology and medicine. Also, modifying the surfaces of nanocrystals can change the electrical and optical properties of such nanocrystals.
Surface modification of nanocrystals using (—SH) is known. While effective in some instances, it would be desirable to provide for other types of ligand reactions, which may be used to produce different types of nanocrystals. Nanocrystals with different types of ligands could be advantageously used in various applications (e.g., electronics).
These and other problems are addressed individually and collectively by embodiments of the invention.